Drill bit shaft structure for excavation apparatus

ABSTRACT

The drill bit shaft structure for an excavation apparatus has a thin-walled shaft portion formed on a hollow drill bit shaft. The thin-walled shaft portion has a thinner wall than that of shaft portions of the hollow drill bit shaft adjacent to both ends of the thin-walled shaft portion. A cylindrical body provided with an adjustable joining structure is disposed so as to envelop the thin-walled shaft portion. Both ends of the cylindrical body are connected to the shaft portions of the hollow drill bit shaft adjacent to the both ends of the thin-walled shaft portion. The direction of excavation is controlled by bending the thin-walled shaft portion and the cylindrical body.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an excavation apparatus used for theexcavation of oil wells, gas wells, and the like, and more particularlyto a drill bit structure that can easily bend in order to change thedirection of excavation.

2. Description of the Related Art

An easily bendable drill bit shaft is necessary for an excavationapparatus in which the drill bit shaft (drill pipe) bends to change thedirection of excavation. However, the drill bit shaft must transmitexcavation torque to the drill bit at the distal end, bear apredetermined axial load, and allow excavation liquid (muddy water) topass through the hollow space thereof. A thick-walled shaft having highbending rigidity is therefore used by necessity as the drill bit shaft.Significant bending stress is produced when a highly rigid drill bitshaft is bent. Therefore, a drill bit shaft cannot be bent to asignificant degree in order to prevent the rotating bending fatiguelimit from being exceeded.

It is proposed in JP-A 09-217576 to provide a joint part such as auniversal joint to the connecting portion of a drill bit. In JP-A08-270369, a drill bit shaft is connected via a flexible joint having apredetermined length, and bending-induced excessive stress is preventedfrom acting on the drill bit shaft.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a drill bit shaftstructure for an excavation apparatus in which the drill bit shaft canbe bent without producing high bending stress.

The drill bit shaft structure for an excavation apparatus according tothe present invention, comprises:

a hollow drill bit shaft;

a thin-walled shaft portion formed on the hollow drill bit shaft, whichhas a predetermined length along an axial direction of the hollow drillbit shaft, and which has a thinner wall than that of shaft portions ofthe hollow drill bit shaft adjacent to both ends of the thin-walledshaft portion; and

a cylindrical body provided with an adjustable joining structure anddisposed so as to envelop the thin-walled shaft portion; wherein

the cylindrical body has two end parts connected to the shaft portionsof the hollow drill bit shaft adjacent to the both ends of thethin-walled shaft portion; and

the direction of excavation is controlled by bending the thin-walledshaft portion and the cylindrical body.

An Oldham coupling structure can be used as the adjustable joiningstructure of the cylindrical body. Alternately, a joint formed bywrapping metal wire of a predetermined thickness in multiple layers in acoil shape may also be used.

It is preferable for the inside diameter of the thin-walled shaftportion to be equal to that of the shaft portions on both sides, and forthe outside diameter of the thin-walled shaft portion to be less thanthat of the shaft portions on both sides. This allows the diameter ofthe hollow part of the drill bit shaft to remain constant, and fluid toflow smoothly through the hollow part. A gap is formed between theexternal peripheral surface of the thin-walled shaft and the internalperipheral surface of the cylindrical body that surrounds the shaft.Therefore, the thin-walled shaft and the cylindrical body can beprevented from coming into contact with each other while in a flexedstate.

A shock-absorbing material comprising heat-resistant rubber or otherresin material may be disposed between the thin-walled shaft and thecylindrical body in order to reliably prevent contact between thethin-walled shaft and the cylindrical body.

In the drill bit shaft structure for an excavation apparatus accordingto the present invention, a thin-walled shaft having a predeterminedlength is formed in part of the drill bit shaft, making it easier tobend the drill bit shaft. In addition, a reduction in the transmissionof excavation torque or the like due to the formation of the thin-walledshaft can be compensated for with a cylindrically configured adjustablejoining structure disposed so as to envelop the thin-walled shaft.

It is therefore possible to use a thick-walled, sufficiently rigid drillbit shaft for the entire structure, and the thin-walled shaft envelopedin the cylindrical body can easily bend without producing excessivestress. The transmission of excavation torque and the like can bemaintained by the cylindrically configured adjustable joining structure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic partial structural view of the excavationapparatus used in the present invention;

FIG. 2 is an explanatory drawing showing the flexible shaft portion ofthe drill bit shaft;

FIG. 3 is an explanatory drawing showing another example of thecylindrical body; and

FIG. 4 is an explanatory drawing showing the flexible shaft portionprovided with a shock-absorbing material.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the drill bit shaft structure for an excavation apparatusin which the present invention was used will now be described withreference to drawings.

FIG. 1 is an explanatory drawing showing the entire structure of theexcavation apparatus. The excavation apparatus 1 has a drill stringconnected to a drill bit shaft 2, a drill collar 3 is connectedcoaxially to the distal end of the drill bit shaft 2, and a drill bit 4is supported on the distal end of the drill collar 3. A rotation driveapparatus (not shown) for rotating the drill string is disposed towardthe back end of the drill string.

The drill bit shaft 2 that constitutes part of the drill string isrotatably supported so as to be coaxial to an excavation hole 5 by aplurality of support bearing mechanisms 6 disposed at predeterminedintervals along the excavation hole 5. At a position along the drill bitshaft 2, the drill bit shaft 2 is supported in a rotatable state by aneccentric support bearing mechanism 7 that is in an offset position fromthe support positions provided by the other support bearing mechanisms6.

The area of the drill bit shaft 2 supported by the eccentric supportbearing mechanism 7 is in an offset position from the areas supported onthe two sides thereof. Therefore, the drill bit shaft 2 is bent as anentire structure, and the shaft portion that protrudes toward the distalend from the support bearing mechanisms 6 is extended in a slightlyinclined direction relative to a central axis 5 a of the excavation hole5. Accordingly, the excavation direction of the drill bit 4 that isattached toward the distal end of the drill bit shaft 2 is inclined inrelation to the central axis 5 a.

As an entire structure, the drill bit shaft 2 is a hollow shaft having aconstant wall thickness, and a flexible shaft portion 10 that is easilybent compared to other portions is formed in a part thereof. Theflexible shaft portion 10 of the drill bit shaft 2 is bent to asignificant degree by the eccentric support bearing mechanism 7.

FIG. 2 is a schematic partial cross-sectional view showing the flexibleshaft portion 10 of the drill bit shaft 2. Within the flexible shaftportion 10, part of the hollow drill bit shaft 2 is made into athin-walled shaft portion 21 having thinner walls than shaft portions22, 23 on both sides over a predetermined length, and a cylindrical body30 provided with an adjustable joining structure is coaxially disposedso as to envelop the thin-walled shaft portion 21.

The wall thickness of two end parts 21 a, 21 b of the thin-walled shaftportion 21 gradually increases and ultimately becomes contiguous withadjacent portions 22 a, 23 a of the shaft portions 22, 23 on both sides.Also, the inside diameter of the thin-walled shaft 21 is equal to theinside diameter of the shaft portions 22, 23 on both sides, and a hollowpart 2 a having a constant inside diameter extends through the drill bitshaft 2. Therefore, the outside diameter of the thin-walled shaftportion 21 is less than that of the shaft portions 22, 23 on both sides,and has a proportionately thinner wall.

The cylindrical body 30 is longer than the thin-walled shaft portion 21,and the two end parts thereof are contiguously fixed to the adjacentportions 22 a, 23 a of the shaft portions 22, 23 on the two sides of thethin-walled shaft portion 21. The inside diameter and outside diameterof the cylindrical body 30 are constant, respectively, and a ring-shapedgap 24 is formed between the internal peripheral surface 31 of thecylindrical body and the external peripheral surface 21 c of thethin-walled shaft portion 21.

The cylindrical body 30 is provided with an adjustable joiningstructure. In the present example, an Oldham coupling structure isprovided, and a plurality of Oldham rings having the same diameter areconnected in a coaxial state. More specifically, the two end parts ofthe cylindrical body 30 are defined by wide Oldham rings 32, 33, andOldham rings 34, 35 are alternately connected in a coaxial statetherebetween. Oldham rings 34 are provided with a pair of key grooves 34a, 34 a formed at point-symmetrical positions on the circular end faceon one side thereof, and a pair of key grooves 34 b, 34 b formed atpoint-symmetrical positions on the circular end face on the other side.The key grooves 34 a, 34 a and the key grooves 34 b, 34 b are formed inpositions offset at 90 degrees. The other Oldham rings 35 are providedwith a pair of keys 35 a, 35 a formed at point-symmetrical positions onthe circular end face on one side thereof, and a pair of keys 35 b, 35 bthat are formed at point-symmetrical positions on the circular end faceon the other side. The keys 35 a, 35 a and the keys 35 b, 35 b areformed in positions offset at 90 degrees. The Oldham rings 34, 35 areconnected so as to be able to move a short distance in a radialdirection relative to each other.

In this arrangement, the direction of rotation of the drill bit shaft 2is constant. Therefore, to prevent backlash in the direction of rotationin the Oldham coupling structure, the Oldham rings can be shifted inadvance in the direction of relative rotation and connected without anygaps so that no backlash occurs in the direction of rotation of thedrill bit shaft 2.

The flexible shaft portion 10 composed of the thin-walled shaft portion21 and the cylindrical body 30 can bend easier than the other shaftportions 22, 23 in the drill bit shaft 2 provided with the flexibleshaft portion 10 constituted in this manner. Therefore, high bendingstress can be prevented from acting on the drill bit shaft 2. Theexcavation torque of the thin-walled shaft portion 21 is transmittedless efficiently than that of the other shaft portions 22, 23, but thereduction in the transmission of excavation torque or the like iscompensated for by the cylindrical body 30, and the same stresstransmission characteristics as those of the other shaft portions 22, 23can be maintained.

FIG. 3 is an explanatory view showing another example of the cylindricalbody. The cylindrical body 40 shown in this drawing is formed into acylinder by wrapping metal wire of a predetermined thickness in multiplelayers in a coil shape.

In this configuration, a cylindrical shock absorber composed ofheat-resistant rubber or other resin material may be disposed betweenthe thin-walled shaft portion 21 and the cylindrical body 30, as shownin FIG. 4. In this manner, contact between the thin-walled shaft portion21 and the cylindrical body 30 can be prevented.

1. A drill bit shaft structure for an excavation apparatus, comprising:a hollow drill bit shaft; a thin-walled shaft portion formed on thehollow drill bit shaft, which has a predetermined length along an axialdirection of the hollow drill bit shaft, and which has a thinner wallthan that of shaft portions of the hollow drill bit shaft adjacent toboth ends of the thin-walled shaft portion; and a cylindrical bodyprovided with an adjustable joining structure and disposed so as toenvelop the thin-walled shaft portion; wherein the cylindrical body hastwo end parts connected to the shaft portions of the hollow drill bitshaft adjacent to the both ends of the thin-walled shaft portion; andthe direction of excavation is controlled by bending the thin-walledshaft portion and the cylindrical body.
 2. The drill bit shaft structurefor an excavation apparatus according to claim 1, wherein the adjustablejoining structure of the cylindrical body is an Oldham coupler.
 3. Thedrill bit shaft structure for an excavation apparatus according to claim1, wherein the adjustable joining structure of the cylindrical body isformed by wrapping metal wire of a predetermined thickness in multiplelayers in a coil shape.
 4. The drill bit shaft structure for anexcavation apparatus according to claim 1, wherein: an inside diameterof the thin-walled shaft portion is equal to that of the remaining shaftportion of the hollow drill bit shaft, and an outside diameter of thethin-walled shaft portion is less than that of the remaining shaftportion of the hollow drill bit shaft.
 5. The drill bit shaft structurefor an excavation apparatus according to claim 4, wherein a shockabsorber comprising heat-resistant rubber or other resin material isdisposed between the thin-walled shaft portion and the cylindrical body.